A homologous gene of S-phase kinase-associated protein1 (SKP1) was cloned from tree peony (Paeonia suffruticosa) and denoted as PSK1. The 462-bp open reading frame of PSK1 was predicted to encode a protein comprising 153 amino acids, with a molecular mass of 17 kDa. The full-length gene was 1,634 bp long and included a large 904-bp intron. PSK1 transcription was detected in all tissues, with the highest level observed in sepals, followed by leaves. Under salinity stress, overexpression of PSK1 in Arabidopsis resulted in increased germination percentages, cotyledon greening, and fresh weights relative to wild-type plants. Furthermore, transgenic Arabidopsis lines containing 35S::PSK1 displayed increased expression of genes that would be essential for reproduction and growth under salinity stress: ASK1, LEAFY, FT, and CO involved in flower development and flowering time as well as P5CS, RAB18, DREB, and SOD1-3 contributing to salinity tolerance. Our functional characterization of PSK1 adds to global knowledge of the multiple functions of previously explored SKP1-like genes in plants and sheds light on the molecular mechanism underlying its role in salinity tolerance. Our findings also provide information on the function and molecular mechanism of PSK1 in tree peony flower development, thereby revealing a theoretical basis for regulation of flowering and conferral of salinity tolerance in tree peony.